1. Field of the Invention
The present invention is broadly concerned with catalysts for the abatement of pollutants, especially the abatement of NO.sub.x and CO. More specifically, the present invention is concerned with a novel catalytic material which comprises a catalytic component and an oxygen storage component comprising a mixed oxide of cerium and zirconium.
2. Related Art
It is well-known in the art to utilize oxidation catalysts, including those commonly referred to as three-way conversion catalysts ("TWC catalysts") to treat the exhaust gases of internal combustion engines. Oxidation catalysts promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") in engine exhaust to H.sub.2 O and CO.sub.2. TWC catalysts promote such oxidation reactions and also promote the substantially simultaneous reduction to N.sub.2 of nitrogen oxides ("NO.sub.x ") in the exhaust. It is well-known that successful functioning of the TWC catalyst to promote oxidation of HC and CO and to substantially simultaneously reduce NO.sub.x requires that the engine be operated at or close to stoichiometric air/fuel conditions.
It is also well-known in the art to provide such catalysts in the form of a catalytic material comprising a refractory inorganic oxide support material, e.g., activated alumina, on which is dispersed a catalytic metal component such as one or more platinum group metal components. The refractory inorganic oxide preferably has a high surface area to enhance the effectiveness of the catalytic metal component dispersed thereon. The catalytic material is normally provided as a thin coating or "washcoat" adhered to the walls of a refractory carrier substrate. The latter often takes the form of a body made from a suitable material such as cordierite, mullite or the like, which is formed to have a plurality of parallel, fine gas flow passages extending therethrough. Typically, there may be from about 150 to 450 or more such gas flow passages per square inch of end face area of the substrate.
Ceria is known to be a useful additive in oxidation catalysts and especially TWC catalysts, in which it is believed to serve as an oxygen reservoir and is sometimes referred to as an oxygen storage component. It is believed that, with the engine operating at air-to-fuel ratios which fluctuate above and below stoichiometric, the ceria takes up oxygen during lean (relatively oxygen-rich) periods of operation and makes oxygen available for the oxidation of hydrocarbons during rich (relatively oxygen-deficient) periods of operation.
At elevated temperatures both activated alumina and bulk ceria are subjected to phase change of their crystalline structures which lowers their surface area and significantly reduces the effectiveness of the catalyst. It is known to stabilize refractory inorganic oxides such as alumina and ceria against such thermal degradation. One known stabilization technique is to impregnate into bulk alumina a solution of a soluble rare earth metal salt, e.g., a cerium salt such as cerium nitrate, and then calcine the impregnated alumina to provide a ceria-impregnated alumina to stabilize the alumina against thermal degradation. It is similarly known to stabilize bulk ceria against thermal degradation by impregnating it with a solution of a soluble aluminum salt such as aluminum nitrate, followed by calcination to provide an alumina-impregnated bulk ceria. While such impregnation techniques are effective to reduce the effects of thermal degradation, ceria is, nonetheless, subjected to degradation and marked reduction of the efficiency of the catalyst of which it is a part, not only by thermal degradation, but also by poisoning of the catalyst by sulfur compounds, such as sulfur oxides which are engendered in the exhaust being treated from sulfur compounds contained in the fuel being burned.
Combined cerium-neodymium-zirconium oxides are commercially available as oxygen storage components for catalytic materials. Typical commercially available combined oxides contain not more than about 21 percent ceria by weight of ceria plus zirconia, in the combined oxide.
U.S. Pat. No. 4,714,694 to Wan et al, dated Dec. 22, 1987, discloses a catalyst material comprising aluminum-stabilized ceria as a support for platinum. The aluminum is impregnated into the bulk ceria to prevent deterioration of catalyst performance after exposure to high temperature aging.
U.S. Pat. No. 5,057,483 to Wan, dated Oct. 15, 1991, discloses a dual-layer catalyst comprising two coats of catalytic material. The first coat comprises platinum dispersed on alumina and bulk ceria. The second coat comprises rhodium dispersed on a co-formed rare earth metal-zirconium oxide and platinum dispersed on alumina. The co-formed rare earth metal-zirconium oxide may comprise one or more oxides of cerium, neodymium and/or yttrium (col. 4, lines 12-15) and contains from about 2 to 30 percent by weight rare earth metal oxides, the balance being predominantly or entirely zirconia (col. 8, lines 43-47). It was hitherto believed that if the quantity of ceria in the co-formed support exceeded about 10 mol percent, i.e., about 30 weight percent, the ceria would not remain in solid solution with the zirconia and the interaction between rhodium and ceria, which adversely affects catalytic performance, would increase to an unacceptable degree. Such interaction occurs mainly when the rhodium is dispersed directly on the ceria, but some such adverse reaction also occurs even when the rhodium is dispersed on other support particles, e.g., on alumina, that are intermixed with the ceria to prepare the catalytic material.
WIPO International Publication WO 95/35152, published Dec. 28, 1995, discloses a catalytic material comprising an oxygen storage composition that comprises ceria and zirconia in amounts of from 1 to 99 percent ceria, preferably 10 to 30 percent, based on the weight of ceria plus zirconia. There may also be an additional rare earth oxide in an amount of from 0.1 to 10 weight percent.
EPO Publication Number 0 507 590 A1, published Oct. 7, 1992, discloses a catalyst composition that may include ceria with zirconia in the form of a composite or solid solution with the weight ratio of the cerium oxide to the zirconium oxide being in the range of 100:2 to 100:60 (see page 3, lines 45-49), indicating a minimum of at least 62.5% ceria. The ceria is said to improve the durability of the catalyst (see page 4, lines 8-9), and there is no recognition of any activity as an oxygen storage component.
U.S. Pat. No. 5,075,276, issued Dec. 24, 1991 to M. Ozawa et al, discloses a catalyst containing ceria as an oxygen storage component which is said to be useful for purification of exhaust gases. The Ozawa et al catalyst comprises a support substrate on which is disposed a washcoat comprising (a) a high surface area material which may be aluminum oxide or titanium oxide, (b) cerium oxide, (c) zirconium oxide and (d) at least one oxide of a rare earth element other than cerium and lanthanum, e.g., neodymium. At least part of the oxides of cerium, zirconium and the rare earth oxide(s) other than lanthanum and cerium is in the form described as a composite or solid solution, which is said to be obtained by either of two methods. One method comprises impregnating the catalyst layer (e.g., platinum on alumina) with three solutions of, respectively, a cerium salt, a zirconium salt and a salt of the rare earth metal, and then "burning" the impregnated catalyst layer at 600.degree. C. or higher (col. 3, line 65 et seq.). The other method comprises mixing the three oxide powders with alumina and "burning" the mixture at 800.degree. C. or higher (col. 4, line 5 et seq.). Catalytic materials such as platinum are then deposited on the oxide layer. Ozawa et al teaches a preference for the first method, citing remarkably superior catalytic performance (col. 4, lines 11-18).